Golf ball

ABSTRACT

A golf ball capable of reducing the moment of inertial while reducing spin in a driver shot will be provided. To that end, a golf ball of the disclosure is a golf ball having a cover and, provided that I b  (g·cm 2 ) represents the moment of inertia of the golf ball, μ (mm) represents deflection hardness corresponding to a deformation amount (mm) of the golf ball in a load direction from when an initial load of 10 kgf is applied to the golf ball to when a final load of 130 kgf is applied to the golf ball, and D represents Shore D hardness of the cover, a spin change amount predictive index ΔS′ represented by the following formula: 
     
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 S 
                 ′ 
               
             
             = 
             
               
                 
                   ( 
                   
                     μ 
                     D 
                   
                   ) 
                 
                 2 
               
                
               
                 
                   
                     82 
                     - 
                     
                       I 
                       b 
                     
                   
                   
                     82 
                     2 
                   
                 
                 · 
                 
                   10 
                   6 
                 
               
             
           
         
       
     
     is at least 2.0.

TECHNICAL FIELD

This disclosure concerns a golf ball.

BACKGROUND

In General, desirable performance of a golf ball (hereinafter, sometimessimply referred to as a “ball”) is such that the ball easily fliesfarther in a driver shot, while the ball easily stops in an approachshot. It has been known that, in order to obtain a ball that easilyflies farther in a driver shot, the spin of the ball in a driver shotshould be reduced, whereas, in order to obtain a ball that easily stopsin an approach shot, the spin of the ball in an approach shot should beincreased.

Conventionally, it had been believed that, in order to reduce the spinin a driver shot, it is effective to increase the moment of inertia ofthe ball, whereas, in order to increase the spin in an approach shot, itis effective to reduce the moment of inertia of the ball (e.g., PLT 1).

CITATION LIST Patent Literature

PLT 1: JP-A-2014-110940

SUMMARY

We have found, however, that, by adjusting the structure of the ballappropriately, it is possible, even when the moment of inertia of theball is small, to not only increase the spin in an approach shot, butalso to reduce the spin in a driver shot.

It could be helpful to provide a golf ball capable of reducing themoment of inertia, while reducing the spin in a driver shot.

A golf ball of the disclosure is a golf ball provided with a cover,wherein,

-   -   provided that        -   I_(b) (g·cm²) represents a moment of inertia of the golf            ball,        -   μ (mm) represents deflection hardness corresponding to a            deformation amount (mm) of the golf ball in a load direction            from when an initial load of 10 kgf is applied to the golf            ball to when a final load of 130 kgf is applied to the golf            ball, and        -   D represents Shore D hardness of the cover,    -   a spin change amount predictive index ΔS′ represented by the        following formula:

${\Delta \; S^{\prime}} = {( \frac{\mu}{D} )^{2}{\frac{82 - I_{b}}{82^{2}} \cdot 10^{6}}}$

is at least 2.0.

The golf ball of the disclosure can reduce the moment of inertia, whilereducing the spin in a driver shot.

The golf ball of the disclosure may be configured such that the cover ismade of urethane.

According to this configuration, the spin in a driver shot can befurther reduced.

The golf ball of the disclosure may be configured such that the spinchange amount predictive index ΔS′ is at least 2.5.

According to this configuration, it is further possible to reduce themoment of inertia, while reducing the spin in a driver shot.

The golf ball of the disclosure may be configured such that the spinchange amount predictive index ΔS′ is at least 3.0.

According to this configuration, it is further possible to reduce themoment of inertia, while reducing the spin in a driver shot.

The golf ball of the disclosure may be configured such that

-   -   the cover is coated with a top coat, and    -   an elastic work recovery rate of the top coat is 30 to 98%.

According to this configuration, the spin in a driver shot can befurther reduced.

The golf ball of the disclosure may be configured such that

-   -   an outer surface of the cover has a plurality of dimples, and    -   provided that        -   PS7 represents an area of the golf ball in contact with a            flat surface upon application of a load of 700 kgf to the            golf ball against the flat surface, and        -   VS represents, assuming that the golf ball has no dimples on            its surface, an area of a circle of a cross-section of the            golf ball taken along a diameter of the golf ball,    -   the following formula:

(PS7/VS/μ)·100≧6.70(mm⁻¹)

is satisfied.

According to this configuration, the spin in a driver shot can befurther reduced.

According to the disclosure, a golf ball capable of reducing the momentof inertia, while reducing the spin in a driver shot, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional diagram illustrating an example of aninternal structure of a golf ball according to one embodiment of thedisclosure;

FIGS. 2A and 2B are diagrams illustrating spins of the golf ball put ina driver shot: FIG. 2A is a schematic diagram illustrating a state of adriver shot, and FIG. 2B is a graph illustrating force acting betweenthe golf ball and a golf club in a driver shot;

FIG. 3 is a diagram illustrating effects of the golf balls of thedisclosure;

FIG. 4 is a diagram illustrating the effects of the golf balls of thedisclosure;

FIGS. 5A to 5F are diagrams illustrating the effects of the golf ballsof the disclosure;

FIGS. 6A to 6F are diagrams illustrating the effects of the golf ballsof the disclosure;

FIGS. 7A and 7B are diagrams illustrating an example of dimplesapplicable to the golf ball of the disclosure: FIG. 7A is a side view ofan example of the golf ball, and FIG. 7B is a cross-sectional view of aportion of the golf ball illustrated in FIG. 7A;

FIGS. 8A and 8B are diagrams illustrating another example of the dimplesapplicable to the golf ball of the disclosure: FIG. 8A is a side view ofanother example of the golf ball, and FIG. 8B is a cross-sectional viewof a portion of the golf ball illustrated in FIG. 8A; and

FIGS. 9A and 9B are diagrams illustrating states of the same golf ballupon applications of respective loads of 6864 N and 1961 N thereto.

DETAILED DESCRIPTION

Hereinafter, embodiments of the disclosure will be described by way ofexample with reference to the drawings.

[Structure of Golf Ball of the Disclosure]

A golf ball according to one embodiment of the disclosure includes, forexample, in addition to a core and an intermediate layer on an outerside of the core, a cover forming an outermost layer.

FIG. 1 is a cross-sectional view illustrating an example of an internalstructure of the golf ball according to one embodiment of thedisclosure. A golf ball 1 of an example of FIG. 1 is what is called afive-piece golf ball including an inner core 11, an intermediate core 12provided on an outer side of the inner core 11, an outer core 13provided on an outer side of the intermediate core 12, an intermediatelayer 14 provided on an outer side of the outer core 13, and a cover 15having a plurality of dimples 30 formed on an outer surface thereof andprovided on an outer side of the intermediate layer 14. The cover 15 iscoated with a top coat 16.

However, the golf ball of the disclosure can have any internal structureother than that of FIG. 1. For example, the core of the golf ball of thedisclosure does not need to have a three-layer structure composed of theinner core 11, the intermediate core 12, and the outer core 13 asillustrated in the example of FIG. 1 but can have a structure composedof one layer, two layers, or 4 or more layers. Also, the intermediatelayer of the golf ball of the disclosure can be composed of a pluralityof layers.

According to the golf ball of the disclosure, provided that I_(b)(g·cm²) represents the moment of inertia of the ball, μ (mm) representsdeflection hardness of the ball, and D represents Shore D hardness ofthe cover, a spin change amount predictive index ΔS' represented by thefollowing formula:

$\begin{matrix}{{\Delta \; S^{\prime}} = {( \frac{\mu}{D} )^{2}{\frac{82 - I_{b}}{82^{2}} \cdot 10^{6}}}} & (1)\end{matrix}$

is at least 2.0 (ΔS′≧2.0).

Here, the moment of inertia of the ball (I_(b)) can be obtained bymeasurement using a moment of inertial measuring apparatus (for example,M01-005 manufactured inertia Dynamics, Inc.). This measuring apparatuscalculates the moment of inertial of the golf ball from a differencebetween a period of vibration when the golf ball is placed on a jig ofthe measuring apparatus and a period of vibration when the golf ball isnot placed.

The deflection hardness μ (mm) of the golf ball corresponds to adeformation amount (mm) of the golf ball in a load direction from whenan initial load of 10 kgf (approx. 98 N) is applied to the golf ball towhen a final load of 130 kgf (approx. 1275 N) is applied to the golfball. The higher the value of the deflection hardness of the golf ball,the softer the golf ball.

The Shore D hardness (D) of the cover is a value obtained by preparing asheet-like test piece with a thickness of 2 mm from a material of thecover and measuring hardness of the sheet-like test piece by using anASTM-D2240 standard durometer “Type D”. The higher the value of theShore D hardness of the cover, the harder the cover.

Note that, as can be seen from the formula (1), in order to have apositive value (larger than zero) of the spin change amount predictiveindex ΔS′, the moment of inertia of the golf ball I_(b) needs to besmaller than 82 g·cm². The value 82 in formula (1) is being used basedon the fact that the moment of inertia of existing common golf balls isapproximately 81 to 82 g·cm². That is, the moment of inertia of the golfball of the disclosure is lower than that of the common golf balls.

Note that, hereinafter, the golf ball having the moment of inertia of 82g·cm² is referred to as a “standard ball.

Also, the golf ball of the disclosure satisfies ΔS′≧2.0 by having thefollowing three factors being appropriately adjusted: the moment ofinertia of the ball I_(b), the deflection hardness of the ball, and theShore D hardness of the cover.

The golf ball according to one embodiment of the disclosure satisfiesweight (45.93 g or less) and an outer diameter (42.67 mm or more)prescribed by USGA and R&A.

As can be understood from the descriptions of Examples and ComparativeExamples set forth below, the golf ball of the disclosure, as comparedwith the standard ball, can reduce the moment of inertia, while, notonly increasing spin in an approach shot but also reducing spin in adriver shot.

[How we Obtained Formula for a Spin Change Amount Predictive Index ΔS′]

As described above, we have found that, depending on the structure ofthe ball, it is possible, even when the moment of inertia is small, tonot only increase the spin in an approach shot but also to reduce thespin in a driver shot. We then conceived that the spin change amountpredictive index ΔS′ defined by the formula (1) allows an evaluation ofan actual spin change amount predictive index.

Here, how we obtained the spin change amount predictive index ΔS′ willbe described with reference to FIGS. 2A and 2B. FIG. 2A is a schematicdiagram illustrating a state of a driver shot, and FIG. 2B is a graphillustrating force acting between the golf ball 1 and a head 2 of a golfclub and generated by a driver shot. In the graph of FIG. 2B, thehorizontal axis represents time, and the vertical axis represents forceexerted to the golf ball 1 from a club face 21 of the head 2 of the golfclub. A “contact period” in FIG. 2B refers to a period in which the ball1 is in contact with the club face 21. As for waveforms in FIG. 2B, awaveform of a solid line is a waveform of the force actually acting onthe ball 1, and a sine-like wave, partially drawn with a broken linesmoothly continuous from the waveform of the solid line, is provided toobtain a recoil period T described later.

As illustrated in FIGS. 2A and 2B, in a driver shot, the force (shearforce) acting on the ball 1 from the club face 21 is generated first ina direction of putting a backspin on the ball 1 (in a positivedirection) and, later, in a direction of putting a topspin, reverse tothe backspin, on the ball 1 (in a negative direction). Here, providedthat F_(b) represents a total sum of the force (impulse) acting on theball 1 in the direction of putting the backspin while the club face 21and the ball 1 are in contact with each other, and F_(top) represents atotal sum of the force (the impulse) generated in the direction ofputting the topspin while the club face 21 and the ball 1 are in contactwith each other (taking positive and negative signs into account), asthe absolute value of the total thereof (F_(back)+t_(top)) becomessmaller, the spin amount of the spin put on the ball 1 by a driver shotdecreases, hence it becomes more favorable.

In the graph of FIG. 2B, T representing the period of the waveformpartially drawn with a broken line smoothly continuous from the waveformof the solid line (also referred to as the “recoil period”) is expressedby the following formula:

$\begin{matrix}{T = {2{\pi/\sqrt{\frac{K_{x}}{m} + \frac{K_{t}}{I}}}}} & (2)\end{matrix}$

Here, K_(x) represents transverse rigidity of the ball, K_(t) representsrotational rigidity of the ball, m represents mass of the ball, and Irepresents the moment of inertia of the ball.

As a result of various experiments and analyses, we have found that:

(i) as the recoil period T becomes shorter, the total sum(F_(back)+F_(top)) of the impulse of the force exerted on the ball 1from the club face 21 becomes smaller, thus the spin amount decreases,

(ii) as the deflection hardness (pi) of the ball 1 becomes higher (i.e.,as the ball becomes softer), the contact period of the club face 2 andthe ball 1 becomes longer, and the total sum of the force (the impulse)F_(top) generated in the direction of putting the topspin on the ballincreases, and hence the spin amount decreases, and

(iii) as the Shore D hardness (D) of the cover of the ball 1 becomessmaller (i.e., as the cover becomes softer), the friction between theclub face 2 and the ball 1 becomes higher, and the shear force exertedon the ball is generated earlier,

and also found the relationships between the points (i) to (iii). Thepoints (ii) and (iii) can increase or decrease the effect of point (i).Based on these findings, we defined an index S for predicting the effectof the spin amounts in a driver shot and an approach shot from thestructure of the golf ball as

$\begin{matrix}{S = {{\frac{\mu}{D} \cdot \frac{2\pi}{T}} = {\frac{\mu}{D}\sqrt{\frac{K_{x}}{m} + \frac{K_{t}}{I}}}}} & (3)\end{matrix}$

The meaning of this spin amount predictive index S is such that, whenball structures of the same deflection hardness μ and the same Shore Dhardness D are compared, where the moment of inertia I is a variablevalue, the larger the spin amount predictive index S, the less the spinamount in a driver shot.

In formula (3), when the moment of inertia of the ball is reduced as

I→I−ΔI,

a change amount ΔS of the spin predictive index S is

$\begin{matrix}{{\Delta \; S} = {{\frac{\partial S}{\partial I}\Delta \; I} = {{\frac{\frac{\mu \; K_{t}}{2\; D}}{\sqrt{\frac{K_{x}}{m} + \frac{K_{t}}{I}}} \cdot \frac{\Delta \; I}{I^{2}}} = {\frac{K_{t}}{2}( \frac{\mu}{D} )^{2}\frac{1}{S}\frac{\Delta \; I}{I^{2}}}}}} & (4)\end{matrix}$

In formula (4), suppose

ΔI=I _(a) −I _(b)

is satisfied,

$\begin{matrix}{{\Delta \; S} = {\frac{K_{t}}{2}( \frac{\mu}{D} )^{2}\frac{1}{S}\frac{I_{a} - I_{b}}{I_{a}^{2}}}} & (5)\end{matrix}$

is obtained. Here, I_(a) represents the moment of inertia of thestandard ball, and I_(b) represents the moment of inertia of the ballsubject to evaluation.

In formula (5), K_(t) and S are values associated with the standard balland thus can be regarded as constant coefficients. For convenience, theconstant coefficients in formula (5) are manipulated as shown in formula(6), whereby a spin change amount predictive index ΔS′ is defined by

$\begin{matrix}{{\Delta \; S^{\prime}} = {{\frac{2}{K_{t}}S\; \Delta \; {S \cdot 10^{6}}} = {( \frac{\mu}{D} )^{2}\frac{1}{S}{\frac{I_{a} - I_{b}}{I_{a}^{2}} \cdot 10^{6}}}}} & (6)\end{matrix}$

In formula (6), if the moment of inertia of the standard ball I_(a) issubstituted by 82, the formula (1) set forth above can be obtained.

EXAMPLES AND COMPARATIVE EXAMPLES

As described above, the golf ball of the disclosure is configured suchthat ΔS′≧2.0 is satisfied by having the following three factors beingappropriately adjusted: the moment of inertia of the ball (I_(b)), thedeflection hardness (μ) of the ball, and the Shore D hardness (D) of thecover. With this configuration, as compared with the standard ball, themoment of inertia is reduced, while, not only increasing the spin in anapproach shot, but also reducing the spin in a driver shot.

The golf balls of the disclosure according to Examples 1 to 13 andComparative Examples 1 to 14 were prepared and evaluated. Results of theevaluation will be described with reference to Tables 1 to 5 and FIGS. 3to 4. Details of Examples 1 to 13 are shown in Table 1, and details ofComparative Examples 1 to 14 are shown in Table 2.

In Table 1 and Table 2, lower case letters a to u shown in columns of“Composition” of the inner core 11, the intermediate core 12, and theouter core 13 correspond to compositions a to u in Table 3,respectively. In Table 1 and Table 2, also, upper case letters A to Hshown in columns of “Composition” of the intermediate core 12, the outercore 13, the intermediate layer 14, and the cover 15 correspond tocompositions A to H in Table 4, respectively. The numbers ofcompositions in Tables 3 and 4 are in unit of parts by weight.

In Tables 1 and 2, “μ: Deflection hardness (mm)” is the deformationamount (mm) of the respective balls in the load direction from when aninitial load of 10 kgf (approx. 98 N) is applied to the ball to when afinal load of 130 kgf (approx. 1275 N) is applied to the ball.

In Tables 1 and 2. “I_(b): Moment of inertia (g·cm²)” is a valueobtained by measuring the respective balls using a moment of inertiameasuring apparatus (M01-005 manufactured inertia Dynamics, Inc.).

In Tables 1 and 2, “Shore D hardness” of the intermediate layer 14 and“D: Shore D hardness” of the cover 15 were obtained by preparing asheet-like test piece with the thickness of 2 mm from respectivematerials and measuring the hardness of the test piece using anASTM-D2240 standard durometer “Type D”.

In Tables 1 and 2, “ΔS′: Spin change amount predictive index” is a valuecalculated from the formula (1) set forth above by using values p, D,and I_(b) of the respective balls.

In Tables 1 and 2. “Driver spin (rpm)” and “Approach spin (rpm)” referto results of experiments of the spin amounts that were obtained by adriver shot and an approach shot, respectively, using respective balls.

In experiments of driver shot, a driver (W#1) was attached to a GolfSwing Robot (manufactured by Miyamae Co., Ltd.), and the spin amount atthe time when the robot hit the ball, with a head speed (HS) of 45 m/s,was measured. The golf club that was used for the experiments was“TourStage X-Drive 705 TYPE415 (2011 model)” (loft: 9.5°) manufacturedby Bridgestone Sports Co., Ltd.

In experiments of approach shot, a sand wedge (SW) was attached to aGolf Swing Robot (manufactured by Miyamae Co., Ltd.), and the spinamount at the time when the robot hit the ball, with a head speed (HS)of 20 m/s, was measured. The golf club that was used for the experimentswas “TourStage X-WEDGE” (loft: 56°) manufactured by BridgestoneCorporation.

The columns “Dimples”, “PS7: Pressured area”, “PS2: Pressured area”.“VS: Virtual area”, “(PS7/VS/μ)·100 (mm⁻¹)”, “(PS2/VS/μ)·100 (mm⁻¹)”,and “Top coat” in Tables 1 and 2 will be described later.

TABLE 1 Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- ample ampleample ample ample ample ample ample ample ample ample ample ample 1 2 34 5 6 7 8 9 10 11 12 13 Inner Diameter (mm) 18.1 18.1 18.1 18.1 18.118.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 core Specific gravity 2.85 2.852.85 1.78 1.78 2.85 2.85 2.85 2.85 2.85 2.85 2.85 2.85 Composition d e fh i e f d e f d e f Inter- Diameter (mm) 29 29 29 — — 29 29 29 29 29 2929 29 mediate Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.960.96 0.96 0.96 core Composition E F G F G E F G E F G Outer Diamter (mm)37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 coreSpecific gravity 0.96 0.96 0.96 1.1 1.1 0.96 0.96 0.96 0.96 0.96 0.960.96 0.96 Composition D E H k l E H D E H D E H Inter- Diameter (mm)41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.0541.05 mediate Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.960.96 0.96 0.96 0.96 0.96 layer Shore D 62 62 62 62 62 62 62 62 62 62 6262 62 hardness Composition CI CI CI CI CI CI CI CI CI CI CI CI CI CoverDiameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.742.7 42.7 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.151.15 1.15 1.15 1.15 D: Shore D 47 47 47 47 47 61 61 47 47 47 47 47 47hardness Composition A A A A A B B A A A A A A μ: Deflection 2.5 3.0 3.53.0 3.5 2.8 3.3 2.5 3.0 3.5 2.5 3.0 3.5 hardness (mm) I_(b): Moment of74.5 74.5 74.5 78.7 78.7 74.5 74.5 74.5 74.5 74.5 74.5 74.5 74.5 inertia(g · cm²) Dimples FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS.FIGS. FIGS. FIGS. FIGS. 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 8A 8A 8A and 7Band 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 8Band 8B and 8B PS7: pressured area 225 265 297 268 301 252 284 222 269290 282 329 365 PS2: pressured area 64 72 89 74 92 70 84 62 76 90 75 91100 VS: virtual area 1432 1432 1432 1432 1432 1432 1432 1432 1432 14321432 1432 1432 (PS7/VS/μ) · 100 (mm⁻¹) 6.28 6.17 5.93 6.24 6.01 6.286.01 6.20 6.26 5.79 7.88 7.66 7.28 (PS7/VS/μ) · 100 (mm⁻¹) 1.79 1.681.78 1.72 1.84 1.75 1.78 1.73 1.77 1.80 20.9 2.12 2.00 Top Composition II I I I I I J J J I I I Coat Film thickness 15 15 15 15 15 15 15 15 1515 15 15 15 (μm) Elastic work 16.3 16.3 16.3 16.3 16.3 16.3 16.3 80.180.1 80.1 16.3 16.3 16.3 recovery rate (%) ΔS

: Spin change 3.2 4.5 6.2 2.0 2.7 2.4 3.3 3.2 4.5 6.2 3.2 4.5 6.2 amountpredictive index Driver spin (rpm) 2821 2665 2515 2858 2674 2639 24452805 2654 2527 2811 2651 2518 Approach spin (rpm) 6319 6169 6019 60635856 5819 5669 6332 6194 6034 6341 6197 6043

indicates data missing or illegible when filed

TABLE 2 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar-Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ativeative ative ative ative ative ative ative ative ative ative ExampleExample Example Example Example Example Example Example Example ExampleExample Example Example Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 InnerDiameter (mm) 18.1 18.1 18.1 — — — 18.1 18.1 18.1 18.1 — — — 18.1 coreSpecific gravity 1.4 1.4 1.4 1.78 1.4 1.4 1.4 2.85 Composition p q r g pq r d Inter- Diameter (mm) — — — — — — — — — — — — — — mediate Specificgravity core Composition Outer Diamter (mm) 37.7 37.7 37.7 37.7 37.737.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 core Specific gravity 1.141.14 1.14 1.17 1.17 1.17 1.1 1.14 1.14 1.14 1.17 1.17 1.17 0.96Composition s t u a b c j s t u m n o D Inter- Diameter (mm) 41.05 41.0541.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05mediate Specific gravity 0.96 0.96 0.96 1.0 1.0 1.0 0.96 0.96 0.96 0.961.0 1.0 1.0 0.96 layer Shore D 62 62 62 62 62 62 62 62 62 62 62 62 62 62hardness Composition C1 C1 C1 C2 C2 C2 C1 C1 C1 C1 C2 C2 C2 C2 CoverDiameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.742.7 42.7 42.7 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.151.15 1.15 1.15 1.15 1.15 1.16 D: Shore D 47 47 47 47 47 47 47 61 61 6161 61 61 61 hardness Composition A A A A A A A B B B B B B B μ:Deflection 2.5 3.0 3.5 2.5 3.0 3.5 2.5 2.3 2.8 3.3 2.3 2.8 3.3 2.3hardness (mm) I_(b): Moment of 80 80 80 82 82 82 78.7 80 80 80 82 82 8274.5 inertia (g · cm²) Dimples FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS.FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A7A 7A 7A 7A and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and7B and 7B and 7B and 7B and 7B and 7B PS7: pressured area 224 267 301222 260 295 221 211 250 285 223 261 292 220 PS2: pressured area 63 75 9363 70 86 62 56 72 83 64 71 85 63 VS: virtual area 1432 1432 1432 14321432 1432 1432 1432 1432 1432 1432 1432 1432 1432 (PS7/VS/μ) · 100(mm⁻¹) 6.26 6.22 6.01 6.20 6.05 5.89 6.17 6.41 6.24 6.03 6.77 6.51 6.186.68 (PS7/VS/μ) · 100 (mm⁻¹) 1.76 1.75 1.86 1.76 1.63 1.72 1.73 1.701.80 1.76 1.94 1.77 1.80 1.91 Top Composition I I I I I I I I I I I I II Coat Film thickness 15 15 15 15 15 15 15 15 15 15 15 15 15 15 (μm)Elastic work 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.316.3 16.3 recovery rate (%) ΔS

: Spin change amount 0.8 1.2 1.6 0 0 0 1.4 0.4 0.6 0.9 0 0 0 1.6predictive index Driver spin (rpm) 3102 2941 2779 3079 2919 2759 31422898 2740 2579 2879 2719 2559 2913 Approach spin (rpm) 6201 6023 58366155 5986 5816 6251 5883 5692 5486 5805 5636 5466 5969

indicates data missing or illegible when filed

TABLE 3 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo-Compo- Compo- Compo- sition sition sition sition sition sition sitionsition sition sition sition sition a b c d e f g h i j k l Polyb

100 100 100 100 100 100 100 100 100 100 100 100 Zinc 27.0 23.0 19.0 20.017.0 14.0 20.0 17.0 14.0 27.0 23.0 19.0 acrylate Peroxide *1 3 3 3 3 3 33 3 3 3 3 3 Tungsten 0 0 0 268 266 264 102 101.8 101.6 0 0 0 Anti-aging0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 agent *2 Zinc oxide 21.022.5 24.0 5.0 5.0 5.0 5. 5.0 5.0 10.0 11.5 13.5 Compo- Compo- Compo-Compo- Compo- Compo- Compo- Compo- Compo- sition sition sition sitionsition sition sition sition sition m n o p q r s t u Polyb

100 100 100 100 100 100 100 100 100 Zinc 25.5 21.5 17.5 20.0 17.0 14.027.0 23.0 19.0 acrylate Peroxide *1 3 3 3 3 3 3 3 3 3 Tungsten 0 0 0 4848.5 49 0 0 0 Anti-aging 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 agent *2Zinc oxide 22.0 23.5 25.0 5.0 5.0 5.0 8.0 10.0 12.0 *1 Peroxide(2):Mixture of 1,1-Di(t-butylperoxy)cyclohexane and silica, product name PER

 (product of NOF Corporation) *2 Anti-aging agent: product name No

 NS-6 (product of Ouchi Shinko Chemical Industrial Co. Ltd)

indicates data missing or illegible when filed

TABLE 4 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo-sition sition sition sition sition sition sition sition sition A B C1 C2D E F G H T-8290 75 — — — — — — — — T-8283 25 — — — — — — — — T-8260 —100 — — — — — — — Himilan 1706 — — 35 35 — — — — — Himilan 1557 — — 1515 — — — — — Himilan 1605 — — 50 50 — — — — — HPF1000 — — — — 100 — — —— HPF2000 — — — — — 100 — — 50 AD1035 — — — — — — 100 — 50 AD1172 — — —— — — — 100 — Hytrel 4001 11 11 — — — — — — — Titanium oxide 3.9 3 — 7 —— — — — Polyethylene 1.2 1 — — — — — — — wax Isocyanate 7.5 7.5 — — — —— — — compound Trimethylol- — — 1.1 1.1 — — — — — propane

The following are details of materials in Table 4.

T-8290: PANDEX® produced by DIC Bayer Polymer Ltd., MDI-PTMG-typethermoplastic polyurethane

T-8283: PANDEX®@ produced by DIC Bayer Polymer Ltd., MDI-PTMG-typethermoplastic polyurethane

T-8260: PANDEX® produced by DIC Bayer Polymer Ltd., MDI-PTMG-typethermoplastic polyurethane

Himilan 1706: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1557: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1605: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

HPF1000: Dupont HPF

HPF2000: Dupont HPF

AD1035: Dupont HPF

AD1172: Dupont HPF

Hytrel 4001: thermoplastic polyether-ester elastomer produced by DuPont-Toray Co., Ltd.

Polyethylene wax: “SANWAX161P” (product of Sanyo Chemical Industries,Ltd.)

Isocyanate composition: 4,4′-diphenylmethane diisocyanate

*Note that C1 and C2 have equivalent physical property values, anddiffer only in specific gravity.

FIG. 3 is a diagram illustrating the driver spin (rpm) and the approachspin (rpm) of the balls of the Examples 1 to 5 and Comparative Examples1 to 7, the covers 15 of which have the Shore D hardness (D) of 47. FIG.4 is a diagram illustrating the driver spin (rpm) and the approach spin(rpm) of the balls of the Examples 6 to 7 and Comparative Examples 8 to14, the covers 15 of which have the Shore D hardness (D) of 61. In FIG.3 and FIG. 4, the horizontal axis represents the driver spin (rpm), andthe vertical axis represents the approach spin (rpm). As describedabove, it is desirable when the spin amount is small in a driver shotand when the spin amount is large in an approach shot. Therefore, inFIG. 3 and FIG. 4, performance of the ball becomes more favorable whilemoving from the lower right towards the upper left.

As can be seen in FIG. 3 and FIG. 4, when comparing the balls of theComparative Examples and Examples which have the covers 15 of the sameShore D hardness (D), the balls of the Examples which satisfied ΔS′≧2.0,as compared with the balls of the Comparative Examples, favorablyachieved both increase of the spin amount in an approach shot andreduction of the spin amount in a driver shot.

FIGS. 5A to 5F each illustrate the driver spin (rpm) of the balls of theComparative Examples and Examples which have the covers 15 of the sameShore D hardness (D) and deflection hardness (μ). FIGS. 6A to 6F eachillustrate the approach spin (rpm) of the balls of the ComparativeExamples and Examples which have the covers 15 of the same Shore Dhardness (D) and deflection hardness (p).

As can be seen in FIGS. 5A to 5F and FIGS. 6A to 6F, when comparing theballs of the Comparative Examples and Examples which have the covers 15of the same Shore D hardness (D) and deflection hardness (pi), the ballsof the Examples which satisfied ΔS′≧2.0, as compared with the balls ofthe Comparative Examples, favorably achieved reduction of the spinamount in a driver shot or increase of the spin amount in an approachshot.

Note that, from the viewpoint of reducing the moment of inertia of theball while reducing the spin amount of a driver shot, the ball 1 of thepresent embodiment has the spin change amount predictive index ΔS′ ofpreferably at least 2.5, more preferably at least 3.0.

From a similar viewpoint, the ball 1 of the present embodimentpreferably satisfies I_(b)≦80 g·cm², 2.0 mm≦μ≦4.5 mm, and D≦65. Morepreferably, the ball 1 of the present embodiment satisfies 72g·cm²≦I_(b)≦79 g·cm², 2.5 mm≦μ≦3.0 mm, and D≦55.

The cover 15 of the ball 1 of the present embodiment is preferably madeof urethane. Therein a driver shot, the frictional force between theball 1 and the club face 21 of the golf club can be increased, hence thespin amount can be further reduced.

[Dimples]

Next, the dimples 30 of the ball 1 of the present embodiment will bedescribed in more detail. The dimples 30 of the ball 1 of the presentembodiment can have any shape. FIGS. 7A to 7B and FIGS. 8A to 8Billustrate different examples of the dimples applicable to the ball 1 ofthe present embodiment.

In the example illustrated in FIGS. 7A to 7B, each dimple 30 is curvedin a convex shape protruding toward the inside of the golf ball.

In the example illustrated in FIGS. 8A to 8B, the bottom surface of eachof the dimples 30, only in the central area thereof, has a shapeprotruding toward the outside of the golf ball. In this case, withoutcompromising original aerodynamic performance of the dimples 30, thepressured area, which will be described later, can be enhanced. Notethat, as illustrated in FIG. 8B, the portion protruding toward theoutside of the ball 1 in the central area of the dimples 30 may have aflat shape in the further central region of the central area. In thiscase, as illustrated in FIG. 8B, the peripheral portion of the flatregion may have a chamfered (rounded) corner, whereby the contact areabetween the ball and the club face 21 at the time when the ball is hitcan be increased, and hence the spin amount in a driver shot can bereduced.

Here, the ball 1 of the present embodiment preferably satisfies:

(PS7/VS/μ)·100≧5.70(mm⁻¹)  (7)

In formula (7), “PS7” represents an area (referred to as the “pressuredarea”) (mm²) of the golf ball contacting with a flat surface when a loadof 700 kgf (approx. 6864 N) is applied to the golf ball against the flatsurface. In formula (7), “VS” represents an area (referred to as a“virtual plane area”) (mm²) of the circle of the cross-section of thegolf ball taken along the diameter of the golf ball, when it is assumedthat the golf ball has no dimples 30 on its surface. In formula (7). “μ”represents the deflection hardness (mm) of the ball 1 described above.

Note that “PS7/VS/μ” in formula (7) is synonymous with “PS7/(VS·μ)”.That is, “μ” in formula (7) is a variable of the denominator.

When the pressured area PS7 of the golf ball upon application of theload in a driver shot by a typical golfer satisfies the above formula(7), the contact area between the ball 1 and the club face 21 of thegolf club increases and, simultaneously, the frictional force betweenthe ball 1 and the club face 21 is enhanced. As a result, the backspinamount in a driver shot can be reduced, and hence the fly distance canbe improved.

Note that, from a similar viewpoint, the ball 1 of the presentembodiment more preferably satisfies the following formula:

(PS7/VS/μ)·100≧6.70(mm⁻¹)  (8)

Also, the ball 1 of the present embodiment preferably satisfies thefollowing formula:

(PS2/VS/μ)·100≧1.70(mm⁻¹)  (9)

In formula (9), “PS2” is the area (referred to as the “pressured area”)(mm²) of the golf ball contacting with a flat surface when a load of 200kgf (approx. 1961 N) is applied to the golf ball against the flatsurface. VS and p are the same as those of the formulas (7) and (8).

Note that “PS2/VS/μ” in formula (9) is synonymous with “PS2/(VS·μ)”.That is, “μ” in formula (9) is a variable of the denominator.

When the pressured area PS2 of the golf ball upon application of theload in an approach shot by a typical golfer satisfies the above formula(9), the contact area between the ball 1 and the club face 21 of thegolf club increases and, simultaneously, the frictional force betweenthe ball 1 and the club face 21 is enhanced. Therefore, the backspinamount in an approach shot can be increased, and hence the ball 1 canstop sooner near its falling point.

Also, when the above formula (9) is satisfied, the total sum of theimpulse (F_(back)+F_(top)) of the force exerted on the ball 1 from theclub face 21 in a driver shot becomes smaller and, simultaneously, thecontact period of the club face 2 and the ball 1 becomes longer.Therefore, the total (the impulse) of the force generated in thedirection of putting the top spin on the ball is increased, thereby thespin amount can be further reduced.

Note that, from a similar viewpoint, the ball 1 of the presentembodiment more preferably satisfies the following formula:

(PS2/VS/μ)·100≧1.90(mm⁻¹)  (10)

In reference to Tables 1 and 2, the balls of the Examples 1 to 10 andComparative Examples 1 to 14 had the dimples 30 in the shape illustratedin FIGS. 7A and 7B, and the balls of the Examples 11 to 13 had thedimples 30 in the shape illustrated in FIGS. 8A and 8B.

The balls of the Examples 1 to 10 and Comparative Examples 1 to 14 eachhad the dimples 30 of six types with different diameters, out of whichthe dimples 30 with a typical diameter of 4.4 mm, as illustrated in FIG.7B, had a depth L of 0.150 mm at its deepest point.

The balls of the Examples 11 to 13 had the dimples 30 of six types withdifferent diameters, out of which the dimples 30 with a typical diameterof 4.4 mm, as illustrated in FIG. 8B, had a depth H of 0.097 mm at itscentral point C, a depth D of 0.131 mm at its deepest point, and,provided that a distance L1 along a virtual extension plane (a chaindouble-dashed line in FIG. 8B) of the peripheral surface of the ball 1from the peripheral edge E to the central point C is 100, a distance L2along the virtual extension plane of the peripheral surface of the ball1 from the peripheral edge E to an adjacent deepest position was 39.Further, the dimples 30 with the typical diameter of 4.4 mm had a radiusof curvature R of 0.5 mm and an edge angle A2 of 10.5°.

In Tables 1 and 2, the pressured areas PS7 and PS2 of each ball weremeasured by the following method. First, a pressure-sensitive sheet (apressure measuring film, PRESCALE for medium pressure produced byFujifilm Corporation) was placed on a flat surface, and the golf ballsof the Examples and Comparative Examples were placed thereon. Then, byusing Model 4204 produced instron Corporation, the load of 700 kgf(approx. 6864 N) and the load of 200 kgf (approx. 1961 N) wereseparately applied to the golf balls, and then the total area where thepressure-sensitive sheet developed color due to contact with the golfball was measured by using PRESCALE pressure image analysis systemFPD-9270 (product of Fujifilm Corporation). The pressured areas PS7 andPS2 in Tables 1 and 2 indicate results of the measurement conducted on arandom portion of the golf ball.

FIG. 9A illustrates an example of an actual pressure-sensitive sheetwhich developed color upon application of the load of 700 kgf (approx.6864 N) to the golf ball, and FIG. 9B illustrates an example of anactual pressure-sensitive sheet which developed color upon applicationof the load of 200 kgf (approx. 1961 N) to the same golf ball as that ofFIG. 9A. In these figures, the circle portions are the dimples 30, andthe colored area is where the color was developed.

[Top Coat]

Next, the top coat 16 coated on the cover 15 of the ball 1 of thepresent embodiment will be described in more detail. For the ball 1 ofthe present embodiment, the method of forming the top coat 16 (a coatinglayer) by coating the outer surface of the cover 15 with a coatingmaterial may be any method including, for example, an air gun coatingmethod, an electrostatic coating method, and the like.

The thickness of the top coat 16 is not particularly limited but isnormally 8 to 22 μm, preferably 10 to 20 μm.

The top coat 16 preferably has an “elastic work recovery rate”, whichwill be described later, of 30 to 98%, more preferably 70 to 90%. Whenthe elastic work recovery rate of the top coat 16 is within the aboveranges, the coating film formed on the surface of the golf ball hashigher self-repair-and-recovery function while maintaining constanthardness and elasticity, thereby contributing to excellent durabilityand abrasion resistance of the ball. However, when the elastic workrecovery rate of the top coat 16 deviates from the above ranges, thereis a risk that sufficient approach spin may not be obtained.

The elastic work recovery rate of the top coat 16 is one of parametersof a nanoindentation method, which is used for evaluating physicalproperties of a coating film, and which is an ultra-micro hardnesstesting method where indentation load is controlled in the order ofmicronewton (μm) and the depth of an indenter at the time of indentationis tracked with the accuracy of nanometer (nm). Although theconventional method could only measure the size of a deformation mark (aplastic deformation mark) corresponding to the maximum load, thenanoindentation method can obtain a relationship between the indentationload and the indentation depth by automatic and continuous measurement.Therefore, unlike the conventional method, there is no individualdifferences in visual measurement of the deformation mark using anoptical microscope, and hence the nanoindentation method is consideredto be able to reliably and highly accurately evaluate the physicalproperties of a coating film. Accordingly, since the coating film on thesurface of the golf ball can be greatly affected by the hitting by thedriver or various golf clubs and can have more than little influence onvarious physical properties of the golf ball, measuring the coating filmof the golf ball more accurately than the conventionally by using theultra-micro hardness testing method enables a very effective evaluation.

For the balls of the Examples and Comparative Examples shown in Table 1and 2, on the outer surface of the cover 15 (the outermost layver)having numerous dimples 30 formed thereon, the coating material waspainted with an air spray gun so as to form the top coat 16 with athickness of 15 μm. In Tables 1 and 2, alphabets I and J in columns ofthe “Composition” of the top coat 16 correspond to Composition I andComposition J in the following Table 5, respectively.

TABLE 5 Costing composition (parts by mass) Composition I Composition JMain Polyol (1) 100.0 — agent Polyol (2) — 100.0 Ethyl acetate 100.060.0 Propylene glycol 40.0 40.0 monomethyl ester acetate Curing catalyst0.03 0.03 Curing Nurate body of 30.5 52.5 agent hexamethylenediisocyanate (1) Modified polyster of 46.8 — hexamethylene diisocyanate(2) Ethyl acetate 42.7 47.5 Mixing molar ratio (NCO/OH) 1.08 1.08*Coating composition A (molar ratio of NCO) ***Nurate body ofhexamethylene diisocyanate (1): Modified polyester of hexamethylenediisocyanate (2) = 0.79:0.29.

Here, synthesis examples of acrylic polyol (1) and (2) in Table 5 willbe described. Note that, in the following description, “parts” means“parts by mass”.

Synthesis Example 1 of Acrylic Polyol

Into a reactor vessel equipped with a stirrer, a thermometer, a coolingpipe, a nitrogen gas introducing pipe, and a dropping device, 1000 partsof butyl acetate was introduced and, while being stirred, heated to 100°C. Into thus obtained butyl acetate, a mixture of 220 parts of acrylicmonomer containing polyester (PLACCEL FM-3 produced by Daicel ChemicalIndustries, Ltd.), 610 parts of methyl methacrylate, 170 parts of2-hydroxyethyl methacrylate, and 30 parts of 2,2′-azobisisobutyronitrilewas dropped over the period of 4 hours. After the dropping, a mixturethus obtained was left to react at the same temperature for 6 hours.After the reaction, 180 parts of butyl acetate and 150 parts ofpolycaprolactone diol (PLACCEL L205AL produced by Daicel ChemicalIndustries, Ltd.) were introduced into a resulting mixture and mixed.Thereby, transparent acrylic polyol resin solution (Polyol (1) in Table5) with 50% solid content, viscosity of 100 mPa·s (25° C.), weightaverage molecular weight of 10,000, and a hydroxyl value of 113 mgKOH/g(solid content) was obtained.

Synthesis Example 2 of Acrylic Polyol

Into a reactor vessel equipped with a stirrer, a thermometer, a coolingpipe, a nitrogen gas introducing pipe, and a dropping device, 1000 partsof butyl acetate was introduced and, while being stirred, heated to 100°C. Into thus obtained butyl acetate, a mixture of 620 parts of acrylicmonomer containing polyester (PLACCEL FM-3 produced by Daicel ChemicalIndustries, Ltd.), 317 parts of methyl methacrylate, 63 parts of2-hydroxyethyl methacrylate, and 12 parts of 2.2′-azobisisobutyronitrilewas dropped over the period of 4 hours. After the dropping, a mixturethus obtained was left to react at the same temperature for 6 hours.After the reaction, 532 parts of butyl acetate and 520 parts ofpolycaprolactone diol (PLACCEL L205AL produced by Daicel ChemicalIndustries, Ltd.) were introduced into a resulting mixture and mixed.Thereby, transparent acrylic polyol resin solution (Polyol (2) in Table5) with 50% solid content, viscosity of 60 mPa·s (25° C.), weightaverage molecular weight of 70,000, and ahydroxyl value of 142 mgKOH/g(solid content) was obtained.

The elastic work recovery rate of the top coat 16 of the ball 1 of theExamples and Comparative Examples in Tables 1 and 2 was measured asfollows. First, from the coating material used for the top coat 16, acoating film sheet with a thickness of 100 μm was prepared. Then, byusing an ultra-micro hardness tester “ENT-2100” produced by ELIONIXInc., the elastic work recovery rate was measured under the followingcondition.

-   -   Indenter: Berkovich indenter (material: diamond, angle α:        65.03°)    -   Load F: 0.2 mN    -   Loading period: 10 seconds    -   Maintaining period: 1 second    -   Unloading period: 1 second

Note that, based on an indention work amount W_(elast) (Nm) due to arestoring deformation of the coating film and a mechanical indentionwork amount W_(total) (Nm), the elastic work recovery rate can becalculated from the following equation.

Elastic work recovery rate=(W _(elast) /W _(total))·100(%)  (11)

1. A golf ball having a cover, wherein, provided that I_(b) (g·cm²)represents a moment of inertia of the golf ball, μ (mm) representsdeflection hardness corresponding to a deformation amount (mm) of thegolf ball in a load direction from when an initial load of 10 kgf isapplied to the golf ball to when a final load of 130 kgf is applied tothe golf ball, and D represents Shore D hardness of the cover, a spinchange amount predictive index ΔS′ represented by the following formula:$\Delta \; {S^{\prime}( \frac{\mu}{D} )}^{2}{\frac{82 - I_{b}}{82^{2}} \cdot 10^{6}}$is at least 2.0.
 2. The golf ball according to claim 1, wherein thecover is made of urethane.
 3. The golf ball according to claim 1,wherein the spin change amount predictive index ΔS′ is at least 2.5. 4.The golf ball according to claim 1, wherein the spin change amountpredictive index ΔS′ is at least 3.0.
 5. The golf ball according toclaim 1, wherein the cover is coated with a top coat, and an elasticwork recovery rate of the top coat is 30 to 98%.
 6. The golf ballaccording to claim 1, wherein an outer surface of the cover has aplurality of dimples, and provided that PS7 represents an area of thegolf ball in contact with a flat surface upon application of a load of700 kgf to the golf ball against the flat surface, and VS represents,assuming that the golf ball has no dimples on its surface, an area of acircle of a cross-section of the golf ball taken along a diameter of thegolf ball, the following formula:(PS7/VS/μ)·100≧6.70(mm⁻¹) is satisfied.